thr_create/pthread_create and other thread functions always return -1

If you use SunPRO C 3.0 or later, you need to specify the
commandline option ``-mt'' when compiling and linking. If you and
earlier version of SunPRO C or when using gcc, you'll need to
specify -D_REENTRANT on compile command lines and -lthread/-lpthread
on the link command line. -lthread/-lpthread should precede -lc.
Compilers implicitly add -lc at the end of the link command line;
you do not have to add it yourself.
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